1. Field of the Invention
The present invention relates to a process and apparatus for the continuous determination of the sylvine saturation of hot solutions for monitoring and controlling the hot solubilizing of a crude potash salt.
2. The Prior Art
The hot dissolution process is a treatment process customarily used in the potash industry for obtaining potassium chloride. In this process, the different dissolution behavior of halite, sylvine and primarily of kieserite is exploited.
The dissolving of the potassium chloride out of the crude salt is carried out with a hot so-called dissolving lye, which contains sodium chloride, potassium chloride, magnesium chloride and magnesium sulfate in different concentrations. In this connection, the solubility conditions are approximately represented by the quinary system: EQU Na.sup.+, K.sup.+, Mg.sup.2+, Cl.sup.- and SO.sub.4.sup.2- ; H.sub.2 O
(Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 22, 5th Edition, Weinheim: Chemie, 1993, p. 53).
In this process, the dissolving lye quantity, which is maintained in circulation, is kept approximately constant, and the crude salt is metered in depending on the content of valuable product. From the hot crude solution obtained in this way, the KCl can be recovered by a fractionated crystallization process. The sylvine saturation degree (S.D.) of the crude solution is decisive for the amount of the potassium chloride yield. In the present case, the "S.D." is to be understood as follows: The relation between the KCl-concentration of the exclusively halite-saturated crude solution and a comparable solution additionally saturated with sylvine. In case the S.D. is too high, the potassium chloride is not completely dissolved out of the crude salt. This leads to losses of valuable product via the residue.
In case the S.D. is too low, the potassium chloride is dissolved out of the crude salt as far as possible; however, less valuable material is obtained per unit of dissolving lye. Furthermore, this leads to an undesirably high dissolution of rock salt.
According to the present state of the art, the hot dissolution process is controlled via physical methods of determination (temperature, density via areometer, radiometric potassium determination), and based on chemical analyses. For optimizing or controlling said process, said determinations are required in the shortest possible time intervals. For the determination of the density and for the chemical analysis of the solution, it is always necessary to obtain solutions that are free of solids. Based on the determinations carried out and the known solubility conditions of. the system specified above, it is possible to calculate the S.D. of the crude salt solution with respect to KCl. Even if the determinations could be automated completely, which has not as yet been realized worldwide in the potash industry, the result would hardly satisfy. The errors of all determinations required here, as well as the errors of the respective equilibrium values lead according to the error propagation law to a high inaccuracy of the computed value of the saturation degree.
In the trade journal "Z. angew. Phys." 27 (1969), No. 4, p. 273, O. Kratky, H. Leopold and H. Stabinger describe the use of highly sensitive sensors that are suitable for the determination of the concentration or density of gases and liquids. This measurement is based on the fact that a defined volume of the measured material participates in the undamped oscillation of a bending oscillator, whose natural frequency is changed by such participation.